hardening off in greehouse

I've just grown my 1st lot of bedding plants from seed stuff like marigold lobelia, busy Lizzy, cosmos. I started them off in a new heated propagator in the polytunnel. I desperately need to prick them out into larger pots now as they are all getting a bit leggy and then they won't fit into the propagator anymore, so will have to go into the polytunnel as they will take up to much space. Its got a frost heater in there set to about 3 or 4C. My question is will that be warm enough or should I increase the temperature in there for a few night to get them acclimatised. The propagator was set to 15C, and I've been opening it during the day and the poly tunnel doors if the suns out. We've got some warmer weather coming, but lows of 4 tonight. What are your thoughts?

 

It kind of depends what they are, most of those will take lower temperatures once acclimatised and for short periods.
Taking straight from a propagator to conditions out of it can be tricky until they harden up if soft.
Can you switch off the propagator and take the lid off for a few days first before moving on.

 

I would leave the frost heater setting alone, as Pete says take the lid off during the day and I would cover with some garden fleece during the day. At night fleece off, lid back on if cold night cover propagator with fleece or a few sheets of newspaper. After a couple of days I'd take the lid off all the time leave covered with fleece, add more fleece or newspaper on cold nights then if overcast take the fleece off during the day.

 

You might consider creating a' hotbed' area using a soil warming cable with a time-switch and thermostat option . An 'intensive care ' ward,or extensive propagator !
Such as this:
Soil Warming / Heating Cable 3m-48m Optional Thermostat | Heated Bed Propagator | eBay
Ideally create a tray to fit multiples of your growing trays. Fill tray with wet sand over the cable laid out in grid form. It needs about two hours to reach suitable temperatures so offset times to reflect. A lightweight frame to support bubble wrap, for extra cold nights. I would regulate to max 12 deg C for economy, with a set minimum of ,say 6 deg

 

I've moved all my germinated seedlings out permanently into our unheated greenhouse now. Most nights they are returned to the bubble wrapped enclosure side of the greenhouse and in addition to this ( and depending on the temperatures) I add another covering of fleece or/ and very lightweight foam sheeting.

Last night they were left out of the enclosure as it was so mild...all were fine this morning.

It's the only real way that I can ensure they get maximum daylight hours, otherwise they can suddenly get leggy and weak.

What part of the country are you in?

It means that I have to keep a very close eye on our forecast and have a good supply of varying types of frost/cold coverings to hand.....but I think we all do that at this time of year.

Good luck James.